1. Field of the Invention
This invention relates to a laser beam scanning system for deflecting a laser beam to scan a surface-to-be-scanned, and more particularly to such a laser beam scanning system which is less affected by wandering of the laser source.
2. Description of the Prior Art
Recently, there have been developed various laser beam scanning systems for reading and/or recording of an image by the use of a laser beam. In such systems, a laser beam emitted from a laser source is deflected by a light deflector such as a rotational polygonal mirror or a galvanometer mirror to scan a surface-to-be-scanned which is conveyed at a constant speed in a sub-scanning direction, i.e., a direction perpendicular to the deflecting direction.
The beam emanating position and the beam emanating direction can vary from laser source to laser source, and at the same time, the beam emanating position and the beam emanating direction of a laser source can be changed when environmental conditions such as startup time, temperature, environmental stress and the like change. The latter phenomenon is generally referred to as "laser beam wandering", and when laser beam wandering occurs, the optical path of the laser beam fluctuates and the scanning accuracy is lowered. Particularly significant is the fact that fluctuation in the beam emanating position is enlarged by the beam expander which is generally provided between the laser source and the light deflector and a part of the laser beam can be prevented from impinging upon the deflecting surface of the light deflector. Further, when wobbling of the light deflector shaft occurs in the light deflector for deflecting the laser beam, scanning lines on the surface-to-be-scanned can be distorted in the sub-scanning direction. In order to avoid this, there has been proposed a scanning system in which an anamorphic optical system is provided between the light deflector and the surface-to-be-scanned. However, in such a scanning system, the problem of laser beam wandering is particularly serious. That is, in the scanning system, an incident optical system such as a cylindrical lens is disposed between the laser source and the light deflector to cause the laser beam to impinge upon the deflecting surface of the light deflector as a line image extending perpendicular to the driving shaft of the light deflector, and an anamorphic optical system such as a cylindrical lens is provided between the light deflector and the surface-to-be-scanned to make the deflecting surface of the light deflector and the surface-to-be-scanned conjugate to each other and to cause the laser beam in the form of the line image to focus on the surface-to-be-scanned as a point image, thereby causing the laser beam to fixedly focus on the surface-to-be-scanned in the regular position irrespective of wobbling of the light deflector shaft. In the scanning system, the cylindrical lens between the light deflector and the surface-to-be-scanned makes it difficult to cause the laser beam to precisely focus on the surface-to-be-scanned, and curvature of field, i.e., arcing of the locus of the focusing point, is apt to occur, whereby the laser beam can blur at the ends of the scanning range. Accordingly, if the beam emanating position and the beam emanating direction fluctuate in the main scanning direction, the optical path of the laser beam after deflection can deviate to increase the curvature of field and at the same time, the accuracy of shaft wobbling correction can be lowered.
Though the problems described above can be overcome, for instance, by a careful selection of the laser source or by strictly controlling the temperature near the laser mount, these approaches will substantially add to the cost.